1. Field of the Invention
The present invention relates to high temperature aluminum bearing alloy coatings commonly referred to as overlay coatings and to their application to superalloy substrates.
2. Description of the Prior Art
It is well known that the family of high temperature, oxidation-corrosion resistant coatings generally referred to as MCrAlY coatings can markedly extend the service life of gas turbine blades, vanes and like components; for example, see U.S. Patents to Evans et al, U.S. Pat. No. 3,676,085; Goward et al, U.S. Pat. No. 3,754,903; Hecht et al, U.S. Pat. No. 3,928,026; and Talboom, Jr. et al, U.S. Pat. No. 3,542,530, all of which are of common assignee herewith. These MCrAlY coatings are referred to as overlay coatings to denote the fact that they are deposited on the substrate as an alloy and act substantially independently of the substrate in providing oxidation-corrosion protection.
In the past, these types of coatings have been applied by vacuum vapor deposition, sputtering and plasma spraying techniques. Vacuum vapor deposition and sputtering are similar in that coating deposition is effected by exposing the substrate to a metallic vapor, usually in vacuum, the vapor condensing on the substrate surface to form the overlay coating, for example, see Steigerwald, U.S. Pat. No. 2,764,420 and Krutenat, U.S. Pat. No. 3,799,862. A disadvantage associated with the vapor depositing and sputtering of MCrAlY coatings is the formation of so-called leader defects in the coating. These defects are in the form of elongated voids between the columnar grains of the as-deposited coating and are known to reduce coating oxidation-corrosion resistance by providing convenient access paths for penetration of the corrosive environment into the coating. Generally, in the commercial manufacture of MCrAlY coated parts, such as turbine blades and vanes, the coated part is glass bead peened to close the leader defects. Of course, glass bead peening is an additional step in the process and increases the cost of producing an acceptable MCrAlY coated part.
Also, it has been thought highly desirable to provide a more ductile, that is, lower aluminum, MCrAlY layer between the substrate and an outer oxidation resistant MCrAlY layer of normal composition. The theory behind this thought is that the intermediate, lower aluminum layer would function as a ductile barrier zone to blunt and stop thermal cracks which are sometimes generated during high temperature service from propagating to the substrate and thereby exposing it to the harmful environment. However, currently used vapor deposition and sputtering techniques are not adapted to readily produce such coating compositional variations since the metallic vapor to which the substrate is exposed is usually generated from a single ingot source having a homogeneous composition. Although it may be possible to provide an ingot source of heterogeneous composition to deposit such a coating on the substrate, this would appear to be a costly solution due to the numerous steps required to fabricate such an ingot. Of course, two step coating processes utilizing different ingot source compositions can be employed to deposit a ductile MCrAlY layer first and then an oxidation resistant MCrAlY layer but these processes too would appear to be costly solutions.